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United States Patent |
6,184,424
|
Bueschken
,   et al.
|
February 6, 2001
|
Process for the hydrogenation of hydroformylation mixtures
Abstract
A process for the hydrogenation of reaction mixtures from the
hydroformylation of C.sub.5 to C.sub.24 olefins using hydrogen on fixed
catalysts at elevated temperature, in which the aldehydes, alcohols,
formates and low-boilers are evaporated from the reaction mixture and
passed in the vapor state over a support-free Cu/Cr catalyst.
Inventors:
|
Bueschken; Wilfried (Haltern, DE);
Gubisch; Dietmar (Marl, DE)
|
Assignee:
|
Oxeno Olefinchemie GmbH (Marl, DE)
|
Appl. No.:
|
396367 |
Filed:
|
September 15, 1999 |
Foreign Application Priority Data
| Sep 16, 1998[DE] | 198 42 369 |
Current U.S. Class: |
568/882; 568/883; 568/885; 568/907; 568/909; 568/909.5; 568/914 |
Intern'l Class: |
C07C 029/16 |
Field of Search: |
568/882,883,885,907,909,909.5,914
|
References Cited
U.S. Patent Documents
5399793 | Mar., 1995 | Vargas et al. | 568/883.
|
5675045 | Oct., 1997 | Bueschken et al. | 568/881.
|
5728891 | Mar., 1998 | Bueschken et al. | 568/376.
|
5756856 | May., 1998 | Bueschken et al. | 568/462.
|
5831135 | Nov., 1998 | Bueschken et al. | 568/881.
|
5922921 | Jul., 1999 | Unruh et al. | 568/882.
|
Foreign Patent Documents |
931888 | Aug., 1955 | DE.
| |
1 935 900 | Feb., 1971 | DE.
| |
35 42 595 A1 | Jun., 1987 | DE.
| |
0 224 872 | Jun., 1987 | EP.
| |
0 850 905 A1 | Jul., 1998 | EP.
| |
2 322 119 | Mar., 1977 | FR.
| |
Other References
Kirk-Othmer, Encyclopedia of Chemical Technology, Fourth Edition, vol. 17,
pp. 909-919, John Wiley & Sons.
New Syntheses with Carbon Monoxide, Edited by J. Falbe, Springer-Verlag,
Berlin Heidelberg New York 1980, pp. 94-115 and pp. 164-165.
|
Primary Examiner: O'Sullivan; Peter
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed is:
1. A process for the hydrogenation of reaction mixtures from the
hydroformylation of C.sub.5 to C.sub.24 olefins, comprising:
evaporating the aldehydes, alcohols, formates and low-boilers from a
reaction mixture obtained from the hydroformylation of C.sub.5 to C.sub.24
olefins to produce a vapor; and
reacting the vapor with hydrogen in the presence of a support-free Cu/Cr
catalyst.
2. The process of claim 1, wherein the process is carried out continuously.
3. The process of claim 1, wherein, using a demister, entrained droplets of
high-boilers are separated off from the evaporated aldehydes, alcohols,
formates and low-boilers upstream of the hydrogenation.
4. The process of claim 3, wherein the high-boilers produced in the
demister and in the evaporator are worked up to materials of value.
5. The process of claim 3, wherein the cycle gas, after leaving the
demister and upstream of entry into the hydrogenation reactor, is heated
and thermostated.
6. The process of claim 3, wherein the temperature of the cycle gas at the
inlet of the hydrogenation reactor is at least as high as at the outlet of
the demister.
7. The process of claim 1, wherein the catalyst comprises from 25 to 40% by
weight of Cu and from 18 to 30% by weight of Cr, based on the oxidic form
of the catalyst.
8. The process of claim 7, wherein the catalyst comprises up to 20% by
weight of a basic substance.
9. The process of claim 8, wherein the catalyst comprises up to 20% by
weight of an inert or property-modifying substance.
10. The process of claim 1, wherein the hydrogenation is carried out on a
fixed-bed catalyst.
11. The process of claim 1, wherein the hydrogenation is conducted at from
150 to 250.degree. C.
12. The process of claim 1, wherein the hydrogenation is conducted from 160
to 220.degree. C.
13. The process of claim 1, wherein the hydrogenation is conducted
adiabatically.
14. The process of claim 1, wherein the hydrogenation is essentially
conducted isothermically, by controlling the temperature by feeding cold
gas.
15. The process of claim 1, wherein the hydrogenation is conducted at a
pressure of from 1 to 25 bar.
16. The process of claim 1, wherein the hydrogenation is conducted at a
pressure of from 15 to 20 bar.
17. The process of claim 1, wherein the liquid hourly space velocity of the
catalyst is between 0.07 h.sup.-1 and 0.40 h.sup.-1.
18. The process of claim 1, wherein the liquid hourly space velocity of the
catalyst is between 0.12 h.sup.-1 and 0.25 h.sup.-1.
19. The process of claim 1, wherein the hydrogenation is conducted at a
hydrogen:starting material mass ratio of from 35:1 to 07:1.
20. The process of claim 1, wherein the hydrogenation is conducted at a
hydrogen:starting material mass ratio of from 3:1 to 1:1.
21. The process of claim 1, wherein the hydroformylation reaction mixture
is obtained by cobalt-catalyst hydroformylation.
22. The process of claim 21, wherein the hydroformylation reaction mixture
is undistilled.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the hydrogenation of
hydroformylation mixtures from the preparation of higher oxo alcohols by
hydroformylation of the corresponding olefins.
2. Description of the Background
Higher alcohols, in particular those having from 6 to 25 carbon atoms, can
be prepared, as is known, by catalytic hydroformylation (or oxo reaction)
of the olefins having one carbon atom less and subsequent catalytic
hydrogenation of the aldehyde- and alcohol-containing reaction mixtures.
They are predominantly used as starting materials for preparing
plasticizers or detergents.
It is known that, in the catalytic hydroformylation of olefins, reaction
mixtures are formed which, apart from the desired products, i.e. aldehydes
and the corresponding alcohols, depending on the catalyst and the reaction
conditions, can comprise by-products and secondary products, such as
unreacted olefins, saturated hydrocarbons formed from the olefins by
hydrogenation, water, esters of the desired alcohols (e.g. formates),
acetals of the target products aldehyde and alcohol, enol ethers and other
by-products or secondary products. These substances can be subdivided into
low-boilers having a boiling point below the boiling point of the aldehyde
and high-boilers having a boiling point above the boiling point of the
alcohol. In the hydrogenation of the reaction mixtures, from some of the
by-products, such as esters and acetals, the alcohols wanted as target
product are formed, which improves the yield. In particular it is desired
that the formates, which can occur in amounts up to 10% by weight, are
hydrogenated under comparatively mild conditions and particularly at low
pressure using commercially conventional catalysts to give the desired
alcohol (and methanol as by-product).
The catalytic hydrogenation of reaction mixtures which were prepared by
cobalt-catalyzed hydroformylation of olefins having from 2 to 24 carbon
atoms is described, for example, in DE 35 42 595. The hydrogenation is
carried out in two stages. In the first stage, the hydroformylation
mixture is hydrogenated at 150-230.degree. C. and a hydrogen pressure of
10-350 bar with 80-95% conversion on a supported SiO.sub.2 catalyst which
comprises 5-15% by weight of nickel and 2-20% by weight of molybdenum in
the form of molybdenum oxide. In the second stage, the hydrogenation is
completed at 150-230.degree. C. and 10-350 bar hydrogen pressure on a
catalyst whose active mass consists of 55-60% by weight of cobalt, 15-20%
by weight of copper, 4-10% by weight of manganese and 2-5% by weight of
molybdenum in the form of molybdenum oxide and, if appropriate, up to 10%
by weight of activating additives. In the process; the formates and
acetals present in the mixture are converted to the corresponding
alcohols. However, the process has the disadvantage that the hydrogenation
is carried out in two stages and at high pressures--in the example at 250
and 245 bar.
According to U.S. Pat. No. 5,399,793, for the hydrogenation of
cobalt-depleted reaction mixtures, as arise in the hydroformylation of
C.sub.5 -C.sub.12 olefins, Ni/Mo catalysts on Al.sub.2 O.sub.3 or Al.sub.2
O.sub.3.SiO.sub.2 as support materials are used. The complete process
comprises the following individual steps:
(a) cobalt-catalyzed hydroformylation,
(b) cobalt depletion of the reaction mixture,
(c) hydrogenation of the crude reaction mixture at elevated temperature and
at elevated pressure,
(d) production of alcohols having very low amounts of aldehydes by
distillation, and
(e) finish-hydrogenation of the alcohols.
The hydrogenation of stages (c) and/or (e) can be carried out using a
bimetallic, phosphorus-free Ni/Mo hydrogenation catalyst. This
hydrogenation catalyst produces fewer high-boiling by-products than a
corresponding phosphorus-containing catalyst. A disadvantage with this
process is that to prepare an on-specification alcohol which is suitable
for preparing plasticizers, two hydrogenation stages are necessary and
that at least in the hydrogenation stage (b) a relatively high pressure of
1000 psig (about 70 bar) is necessary.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method to hydrogenate
reaction mixtures of the hydroformylation of C.sub.5 to C.sub.24 olefins
under comparatively mild conditions on conventional catalysts having a
high service life in such a manner that the aldehydes and the formates
present as by-products are converted into the desired alcohols.
This object, and others, is, surprisingly, accomplished with a process for
the hydrogenation of reaction mixtures from the hydroformylation of
C.sub.5 to C.sub.24 olefins, comprising:
evaporating the aldehydes, alcohols, formates and low-boilers from a
reaction mixture obtained from the hydroformylation of C.sub.5 to C.sub.24
olefins to produce a vapor; and
reacting the vapor with hydrogen in the presence of a support-free Cu/Cr
catalyst.
BRIEF DESCRIPTION OF THE FIGURE
A more complete appreciation of the invention and many of the attendant
advantages thereof will be readily obtained as the same becomes better
understood by reference to the following detailed description when
considered in connection with the accompanying drawings, wherein:
FIG. 1: A block diagram of the present process.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention provides a number of important
advantages. The high-boilers entrained as droplets remaining in the
evaporator and also advantageously separated off from the evaporated
aldehydes, alcohols, formates and low-boilers are not co-hydrogenated, and
do not therefore burden the hydrogenation stage. They can, for example, be
worked up by cleavage or cracking to provide materials of value. The
aldehydes are hydrogenated to the corresponding alcohols at conversion
rates over 98% in a selectivity of above 99% in only one hydrogenation
stage. The esters present, in particular the formates, are likewise
hydrogenated to the desired alcohols. The hydrogenation can be performed
in the low-pressure range of below 25 bar. A desired side effect is that
the starting olefins present in the reaction mixture are predominantly not
hydrogenated, which enables them to be recirculated to the
hydroformylation reaction.
One advantage of the present invention is that the process may be conducted
at low pressures. In addition, entrained droplets of high-boilers are
separated off from the evaporated aldehydes, alcohols, formates and
low-boilers upstream of the hydrogenation.
The FIGURE shows a block diagram of a plant in which the process according
to the invention may be carried out continuously with recirculation as
cycle gas of the hydrogenation hydrogen. The hydroformylation mixture is
introduced as starting material 1 into the evaporator 2, through which
heated hydrogen 3 passes concurrently. The hydrogen stream 4 loaded with
aldehydes, alcohols, formates and low-boilers is conducted through the
demister 5, and the high-boilers separated off there and those remaining
in the evaporator 2 are taken off continuously or batchwise as high-boiler
fraction 6. The hydrogen stream 7 which is freed from high-boilers and
loaded with aldehydes, alcohols, formates and low-boilers is passed into
the hydrogenation reactor 8, from which exits the hydrogenation mixture 9
which is cooled in the cooler 10. In the product receiver 11, the
hydrogenation mixture separates into hydrogenation product 12 and cycle
gas 13, from which a portion is taken off as exhaust gas 14, in order to
keep the inert gas level to an acceptable height. The hydrogen consumed is
replaced by fresh hydrogen 15.
The starting materials for the hydroformylation are monoolefins having from
5 to 24 carbon atoms and a terminal or middle-position C--C double bond or
mixtures of such monoolefins, such as 1- or 2-pentene, 2-methyl-1-butane,
1-, 2- or 3-hexene, the isomeric C.sub.6 olefin mixture (dipropene)
produced in the dimerization of propene, 3-methyl-1-hexene, 1-octene, the
isomeric C.sub.8 olefin mixture (dibutene) produced in the dimerization of
butenes, 1-nonene, 2-, 3- or 4-methyl-1-octane, the isomeric C.sub.9
olefin mixture (tripropene) produced in the trimerization of propene, 1-,
2- or 3-decene, 2-ethyl- 1-octane, 1-dodecene, the isomeric C.sub.12
olefin mixture (tetrapropene or tributene) produced in the tetramerization
of propene or the trimerization of butenes, 1-tetradecene, 1- or
2-hexedecene, C.sub.16 olefin mixtures (tetrabutene) produced in the
tetramerization of butenes, and olefin mixtures prepared by
cooligomerization of olefins having different carbon numbers (preferably 2
to 4), if appropriate after separating off by distillation into fractions
of identical or similar carbon number. Preferably, mixtures are
hydrogenated which are produced in the hydroformylation of C.sub.5,
C.sub.9, C.sub.12 or C.sub.16 olefin mixtures.
The olefins are hydroformylated in a conventional manner and then give the
starting materials for the hydrogenation process according to the
invention. Rhodium catalysts, or preferably cobalt catalysts, are
therefore employed, with or without complex-stabilizing additives, such as
organic phosphines or phosphites. The temperatures and the pressures can
vary, depending on catalyst and olefin, in broad ranges. A description of
the hydroformylation of olefins is found, for example, in J. Falbe, New
Syntheses with Carbon Monoxide, Springer-Verlag, Heidelberg-New York,
1980, pages 99ff., and in Kirk-Othmer, Encyclopedia of Chemical
Technology, volume 17, 4th edition, John Wiley & Sons, pages 909-919
(1996), each incorporated herein by reference.
The hydroformylation reaction mixtures are preferably first separated from
the catalyst. If a cobalt catalyst was used, this can be achieved by
pressure relief, separating off the aqueous catalyst phase, oxidation of
the cobalt carbonyl compounds remaining in the hydroformylation mixture
with air or oxygen and scrubbing out the resulting cobalt compounds with
water or aqueous acid. Cobalt-depletion processes are well known, see, for
example, J. Falbe, loc. cit., 164, 165 (BASF process); Kirk-Othmer, loc.
cit. and EP-0 850 905 A1, each incorporated herein by reference.
If a rhodium compound served as hydroformylation catalyst, it can be
separated off from the hydroformylation mixture as distillation residue by
means of thin-film evaporation.
The hydroformylation reaction mixtures, preferably separated from catalyst,
generally comprise 3-40% by weight, usually 5-30% by weight, of
low-boilers, in addition 30-90% by weight of aldehydes, 5-60% by weight of
alcohols, up to 10% of formates of these alcohols and from 5 to 15% by
weight of high-boilers. However, the process may also be carried out using
hydroformylation mixtures having a composition other than as described
above.
The aldehydes, alcohols, formates and low-boilers are then evaporated from
the generally, undistilled hydroformylation reaction mixture, which if
appropriate is separated from catalyst. The low-boilers particularly
include unreacted olefins and the corresponding saturated hydrocarbons
formed during the hydroformylation and water. The content of low-boilers
in the reaction mixture varies within the limits mentioned above,
depending on the starting olefin, the reaction conditions and the degree
of conversion of the hydroformylation. The high-boilers which according to
the invention are not to pass to the hydrogenation catalyst comprise,
inter alia, aldolization and/or condensation products of the resulting
aldehydes and also acetals and enol ethers and boil, as
higher-molecular-weight substances, considerably higher than the alcohols,
aldehydes, formates and low boilers.
The conditions under which aldehydes, alcohols, formates and low-boilers
are separated from the high-boilers are considerably dependent on the
carbon number of the starting olefins. The reaction mixture is preferably
separated under the same conditions with respect to temperature and
pressure under which the subsequent hydrogenation is carried out. The
pressure is, therefore, generally below 25 bar. It is preferably from 1 to
25 bar, and in particular from 15 to 20 bar. In the case of reaction
mixtures from the hydroformylation of olefins having from 6 to 12 carbon
atoms (for example octenes which were obtained by dimerizing butanes),
temperatures of, for example, from 150 to 250.degree. C., advantageously
from 160 to 220.degree. C., can be employed. For other hydroformylation
mixtures, the optimum temperature conditions for separating off the
high-boilers can be determined without difficulty by simple trial runs.
To separate the hydroformylation mixtures, use is made of conventional
apparatuses, e.g. thin-film evaporators or falling-film evaporators. In an
one embodiment, the mixture is added to a hydrogen stream of appropriate
temperature. Independently of the evaporation method selected, it is
preferable to free the vapor stream from high-boiler droplets, since in
this manner the service life of the catalyst is increased. Conventional
demisters may be used, in which the velocity of the vapor stream is
decreased, the vapor stream is exposed to the action of centrifugal forces
or the droplets are separated off by impact, e.g. on baffles or screens.
If the hydroformylation mixture was evaporated by introduction into a
heated hydrogen stream, the hydrogen/vapor mixture is conducted over the
catalyst. Otherwise, hydrogen is added to the vapor mixture. The hydrogen
is preferably used in a considerable stoichiometric excess Advantageously,
a hydrogen:starting material mass ratio of from 3.5:1 to 0.7:1, in
particular from 3:1 to 1:1, is employed. The unconsumed hydrogen may be
recirculated.
The catalyst is a support-free Cu/Cr catalyst. It is preferably used as a
fixed-bed catalyst and generally comprises from 25 to 40% by weight of
copper and from 18 to 30% by weight of chromium. The catalyst can comprise
up to 20% by weight of basic substances, such as alkali metal oxides or
alkaline earth metal oxides or alkali metal hydroxides or alkaline earth
metal hydroxides, and other, inert or property-modifying substances in the
same amounts, for example graphite. As used herein, the term
"support-free" refers to the absence of support material which has been
sprayed or impregnated with a solution of the active components or onto
which the active components have been adhesively applied in another
manner. The initially oxidic catalyst is expediently reduced by passing
over hydrogen at elevated temperature, e.g. the hydrogenation temperature,
and then develops its optimum activity. The specified percentages by
weight relate to the oxidic, unreduced form of the catalyst. Suitable
catalysts are, for example, the catalyst E406TU from Mallinckrodt, Erie,
Pa. U.S.A., and the catalyst G99B from Sud-Chemie A G, 80333 Munich. The
catalysts are expediently used in a form which offers a low resistance to
flow, e.g. in the form of granules, pellets or shaped bodies such as
tablets, cylinders, rod extrudates or rings. Temperatures at the catalyst
and the pressure advantageously correspond, as mentioned, to the
conditions under which the aldehydes, alcohols, formates and low-boilers
are evaporated from the hydroformylation mixture.
The optimum temperature in the catalyst bed is preferably determined for a
given hydroformylation mixture by preliminary experiments. In the
hydroformylation mixtures which were obtained by hydroformylation of
olefins having from 6 to 12 carbon atoms, it is, as mentioned, from 150 to
250.degree. C., advantageously from 160 to 220.degree. C. It is expedient
that the temperature of the cycle gas comprising the evaporated portion of
the hydroformylation mixture at the inlet of the hydrogenation reactor is
at least as high as at the outlet of the demister. Advantageously, it is
therefore provided that the cycle gas in this process section can be
heated, expediently can be indirectly heated, and can be thermostated.
The hydrogenation generally proceeds exothermically. The reaction can be
conducted adiabatically with temperature increase. Alternatively, it is
also possible to arrange the hydrogenation essentially isothermically,
i.e., to permit a temperature rise of only up to 10.degree. C. from the
reactor inlet to the exit of the hydrogenation mixture. In the latter
case, the temperature is expediently controlled by feeding cold hydrogen.
The process according to the invention permits high throughputs. The liquid
hourly space velocity (LHSV) of the catalyst is given as the hourly
volumetric flow of the still liquid starting material divided by the
catalyst volume. It depends, inter alia, on the temperature selected and
is generally between 0.07 h.sup.-1 and 0.40 h.sup.-1, in particular
between 0.12 h.sup.-1 and 0.25 h.sup.-1. The residence time of the gas
phase in the catalyst zone essentially depends on the GHSV (gas hourly
space velocity), the temperature selected and the pressure and can be, for
example, between 3 and 30 seconds.
The hydrogenation mixture can, after separating off excess hydrogen, be
fractionated into its constituents by fractional condensation or by
complete condensation and distillation. The non-hydrogenated olefins can
be recovered from the low-boiler fraction, advantageously by distillation,
and recirculated into the hydroformylation. Alternatively, the olefins,
together with the saturated hydrocarbons formed from them in the
hydroformylation or the hydrogenation can be used as raw material for
crackers or for heating purposes. This is the case especially if the
hydroformylation was operated with high conversion of the starting
olefins. The alcohols are produced in a purity of >99%, determined by
gas-chromatographic analysis. The residue can be combined with the
high-boilers, which remained in the evaporation of the aldehydes,
alcohols, formates and low-boilers, and can be worked up together with
them to materials of value. For example, by cracking, olefins can be
produced which in turn can be hydroformylated.
Having generally described this invention, a further understanding can be
obtained by reference to certain specific examples which are provided
herein for purposes of illustration only and are not intended to be
limiting unless otherwise specified.
EXAMPLES
Example 1
As starting material 1, hydroformylated di-n-butene was fed by a metering
pump into the evaporator 2 of a gas-phase hydrogenation apparatus
according to the FIGURE, into a hydrogen stream 3 heated to 200.degree. C.
The hydrogen stream 4 which leaves the evaporator and is loaded with
aldehydes, alcohols, formates and low-boilers and comprises high-boiler
droplets was passed through the demister 5 and, after separating off
high-boilers, was passed as material stream 7 to the top of the
hydrogenation reactor 8. The high-boilers 6 remaining in the evaporator
and separated off in the demister 5 were taken off every 12 h.
The reactor was a steel tube of 38 mm open width, in which 800 ml (=1200 g)
of the catalyst E406TU from Mallinckrodt had been arranged, fixed, in the
form of pellets. The catalyst in its oxidic unreduced form, comprised
42% by weight of CuO, equivalent to 33.55% by weight of Cu
40% by weight of Cr.sub.2 O.sub.3 equivalent to 27.37% by weight of Cr
8% by weight of BaO and
10% by weight of graphite.
The catalyst was reduced by 2500 I(S.T.P.)/h of nitrogen firstly being
passed, at 150.degree. C. and 1 bar, over 800 ml of catalyst. A maximum of
5% of the nitrogen was replaced by hydrogen and the gas stream was
controlled in such a manner that the temperature rise remained below
10.degree. C. After 2 h in each case, the volumetric concentration of the
hydrogen was increased by 5%. After reduction had finally been carried out
using pure hydrogen, the temperature was elevated to 160.degree. C. After
a further 2 h in each case, the temperature was increased each time by
10.degree. C. After a temperature of 190.degree. C. had been reached, the
hydrogen pressure was elevated stepwise to 15 bar under strict temperature
control and the catalyst was kept under these conditions for 12 h.
The hydrogenation mixture was cooled in the cooler 10. The condensed
hydrogenation product 12 was taken off from the receiver 11, the cycle gas
13 recirculated to the evaporator and some of the cycle gas was taken off
as exhaust gas 14. Consumed hydrogen was replaced by fresh hydrogen 15.
The process was carried out under the following conditions:
Starting material fed 160 g/h
Fresh hydrogen fed 62 I(S.T.P.)/h
Cycle hydrogen gas 2500 I(S.T.P.)/h
Exhaust gas 50 I(S.T.P.)/h
High-boilers 3 g/h
Temperature downstream of the evaporator 180.degree. C.
Temperature at the hydrogenation reactor inlet 185.degree. C.
Temperature in the hydrogenation reactor 185.degree. C.
Pressure in the system 16 bar
Hydrogenation product approximately 158 g/h
GC analyses of the starting material and of the hydrogenation product gave
the following values:
Starting material Product
Substance (% by weight) (% by weight)
Isononanals 39.4 0.2
Isononanols 41.8 87.9
Isononyl formates 4.2 <0.1
High-boilers 5.9 2.2
After operation for one week, a steady state was reached and the product
composition remained the same for a period of more than 6 months.
Example 2
When the same experiment was carried out without a demister under otherwise
identical conditions, as soon as after 6 weeks a markedly impaired
hydrogenation performance was observed, recognizable by a higher content
of isononyl formates.
Obviously, numerous modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be
understood that within the scope of the appended claims, the invention may
be practiced otherwise than as specifically described herein.
German Patent Application Serial No. 19842369.1 filed on Sep. 16, 1998, is
incorporated herein by reference.
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